1. Field of the Invention
This invention pertains generally to aerosol generating devices, and more particularly to a high efficiency pneumatic nebulizer, which may be used to deliver medicinal aerosols.
2. Description of the Background Art
Nebulizers of various types have been widely used for inhalation delivery of aerosols containing medication or other constituents to the conductive airways of the lung and the gas exchange regions of the deep lung. Aerosols are relatively stable suspensions of finely divided droplets or solid particles in a gaseous medium, usually in air or oxygen. When inhaled, aerosol particles may be deposited by contact upon the various surfaces of the respiratory tract leading to potential injury, desirable therapeutic action, or planned diagnostic behavior depending on the particular properties of the particles. Inhalable aerosol particles are those with an aerodynamic equivalent diameter between 1 and 5 micrometers.
Due largely to the high permeability of the lung and the copious blood flow, medications depositing in the lung readily enter the blood for action throughout the body, while other medications can directly influence the airway epithelium and effect responses via various airway receptors. Properly generated and formulated aerosols can therefore be helpful in medical treatment. As tracers of airflow or indicators of lung responses, other types of aerosol particles deposited in the lung can also be a valuable diagnostic tool.
A nebulizer produces aerosol of fine particles by breaking a fluid into fine droplets and dispersing them into a flowing stream of gas. The droplet size from a medical nebulizer is considerably smaller than a conventional spray atomizer. Medical nebulizers are designed to convert aqueous solutions or colloidal suspensions to aerosols of fine, inhalable droplets that enter the lungs of a patient during inhalation and are then deposited on the surface of the respiratory airways. Typical pneumatic (compressed gas) medical nebulizers in current use develop approximately 15 to 30 microliters of aerosol per liter of gas in finely divided droplets with volume or mass median diameters in the respirable range of 2 to 4 micrometers.
Medication intended for aerosolization generally consists of a solute (the medication) mixed into a base solution in which the solute may be dissolved. Predominantly water or saline solutions are used with low solute concentrations, typically ranging from 1.0 to 5.0 mg/mL. The delivery of solute to the patient""s airway is the primary purpose of a nebulizer. The delivery of solution is usually of minor therapeutic consequence. The output of a medicinal pneumatic nebulizer, carried by a gas, which powers the nebulizer consists of two principal components: vapor, which is solution in gas form without any solute, and small droplets of solution in liquid state which contain a small amount of medicinal solute.
Previously it was generally assumed in the industry that two pneumatic nebulizers having the same gravimetric output, reservoir solute concentration, and gas flow rate, operating under identical conditions would deliver the same amount of solute or medication. The reasoning behind this assumption was that, at the same temperature, both gas streams exiting each nebulizer would have the same percentage of vapor and small droplets, and that each droplet would have a concentration of solute equal to the concentration of the reservoir. This assumption has recently been shown to be false. Large variances in fact exist in medication delivery efficiency between different nebulizers, with nebulizers currently on the market falling at the low end of the possible efficiency spectrum.
The high percentage of vapor among existing nebulizers is the result of internal loss of large particles, which are typically sprayed onto surfaces within the nebulizer. Aerosol production within a pneumatic nebulizer is not a precisely controlled process. Upon production of respirable aerosol within the nebulizer, there are in addition particles created of various others sizes. Of the liquid entrained by a nebulizer jet, typically 1% or less is converted to respirable aerosol particles, which are able to escape the nebulizer. The remaining 99% assume the form of large aerosol particles which are not able to escape the nebulizer (typically greater than 5 xcexcM) and are sprayed onto the internal surface areas of the nebulizer or eventually fall back into the nebulizer reservoir.
As particles are removed from the air stream of the inside of the nebulizer, solution will evaporate to take its place in the form of vapor. The vapor can come from one of two places: from the wetted surface areas internal to the nebulizer, or from other aerosol particles which are still airborne within the nebulizer. Where the vapor comes from is very important to the performance of the nebulizer. Vapor which comes from other airborne particles within the nebulizer, decreases the size of the particle from which the vapor is emerging, making it more probable that the particle will escape the nebulizer, while at the same time increasing the concentration of solute within the aerosol particle. The result is increased quantity of medicinal solute to the patient. Vapor which is formed by the evaporation of solution deposited on the inside surface of the nebulizer has already deposited its solute onto the nebulizer inside surface resulting in less solute, or medication, being delivered to the patient. Additionally the solution which is evaporated off the inside surfaces of the nebulizer, results in solute adhering to the nebulizer surface where it is lost to the patient permanently.
A typical high performing nebulizer might have an aerosol density exiting the nebulizer of 30 xcexcL of fluid per Liter of gas. In general, the gas used to drive the nebulizer is air or oxygen, each of which are capable of carrying 22 xcexcL of water in vapor form per Liter of gas at standard atmospheric conditions. The difference, 8 xcexcL of fluid per Liter of gas, is the minimum guaranteed amount of aerosol particles that are carrying solute or medication. A large portion of nebulizers currently on the market perform at this low threshold, while the remainder generally operates with only marginal improvement.
Therefore, a need exists for a nebulizer which can provide a high concentration of solute (medication) to the patient by minimizing the vapor which is formed by solution evaporating off the inside walls of the nebulizer and maximizing the amount of vapor which is formed by solution evaporating from other aerosol particles which are still within the nebulizer. The present invention satisfies the need for a high performance nebulizer, while it in addition provides this functionality in a condensed and economically manufactured package.
The present invention generally pertains to a pneumatic nebulizer that is able to deliver a high concentration of medication aerosol for a wide range of flow rates and reservoir sizes. The invention is ideal for delivery of medication, which is being carried in a solution, because it is able to deliver a higher medication to solution ratio than existing nebulizers.
By way of example and not of limitation, the present invention employs a jet of gas flowing at the speed of sound to entrain and shatter a stream of fluid against a hemispherical aerosol amplifier which is proximal to the jet orifice. The sonic jet of gas produces a vacuum with respect to atmospheric pressure causing fluid in the nebulizer reservoir to be drawn into the jet of gas. The jet of gas and entrained fluid mix forming small droplets (10-100 xcexcM) which are traveling at a high rate of speed, due to the sonic jet, and are caused to impact on the aerosol amplifier. The aerosol amplifier may be of a variety of shapes provided that it causes the creation of copious amounts of aerosol particles (0-25 xcexcM). A majority of non-respirable particles are produced which are also too large to escape the nebulizer. After striking the aerosol amplifier, the aerosol stream is still travelling at a high rate of speed. Placed circumferentially around the aerosol amplifier are a number of spray posts which collect the bulk of the spray coming off the aerosol amplifier. The spray posts are wide enough and close enough to each other to cause significant pooling of fluid between them without causing pooling of fluid on the aerosol amplifier. Pooling of liquid between the spray posts is primarily the result of the spray posts being close enough that the surface tension of the liquid tends to fill the gap. The pooled fluid acts as a spray baffle which collects most of the aerosol particles greater than 10 xcexcM. Aerosol particles smaller than 10 xcexcM escape by traveling upward between the aerosol amplifier and the posts. Outside the diameter of the perimeter formed by the posts is a containment baffle. The containment baffle is so shaped and positioned to catch large aerosol and spray which may pass through the posts periodically due to sputtering of the fluid build up around the posts. The result is very small wetted surface area. Fluid build up around the spray posts flows down the secondary and returns directly to the fluid reservoir. Minimizing the wetted surface area within the nebulizer is important because it is directly to proportional to the rate of vapor which is formed from solution evaporating off of the internal geometry of the nebulizer.
The particles in the 0 to 5 xcexcM range, which slipped by the posts and the containment baffle, already have a good chance of escaping the nebulizer. The Particles in the 5 to 10 xcexcM range, which also slipped by the baffling action, provide the means for vapor production while at the same time being shrunk to below 5 xcexcM, which will also enable them to escape the nebulizer to be delivered to the patient. Maximizing the aerosol surface area within the nebulizer is primarily a matter of maximizing the number of particles. It is well-known that a volume of gas can only hold up to a certain maximum limit of aerosol particles. Nearing the maximum limit the occurrence of aerosol particle collisions and particle merging increases so as to effectively create the limit. An ideal particle size and a corresponding optimized cutoff limit for the post geometry can be determined in accordance with this limit that varies dependent on total nebulizer volume and flowrate (residence time), but particle size is typically about 10 xcexcM. Producing such a tremendous amount of aerosol, in the range of 0 to 10 xcexcM, creates a situation wherein the combined surface area of the particles far exceeds the wetted surface area inside the nebulizer such that the vapor exiting the nebulizer is primarily derived from the aerosol particles. The result is a high performance nebulizer that produces a high solute to solution ratio aerosol.
An object of the invention is to provide a pneumatic nebulizer which can deliver high solute to solution ratios.
Another object of the invention is to provide the means to achieve the performance of a large volume nebulizer within the geometry of a small or medium sized nebulizer.
Another object of the invention is to maximize the amount of medication that can be delivered to the patient.
Another object of the invention is to minimize the amount of medication deposited on the inside surfaces of the nebulizer.
Another object of the invention is to baffle out the large aerosol particles directly into fluid flowing into the fluid reservoir.
Another object of the invention is to provide the means to control the particle size of the nebulizer.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein, the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.